1. Field of the Invention
This invention relates to a zoom lens, and in particular to the focusing system thereof and to the so-called inner focus or rear focus system by movement of the lens units other than the lens unit (forward group) positioned most adjacent to the object side in a lens system comprised of a plurality of lens units.
2. Related Background Art
In recent years, various systems other than the so-called forward group axial movement system (wherein the forward group most adjacent to the object side is moved to accomplish the in-focus) have been proposed in order to achieve, high performance and high magnification change in zoom lenses.
In the forward group axial movement in-focus system, the lens unit moved for the in-focus is most adjacent to the object side, and this leads to the tendency of the effective aperture of the in-focus lens unit becoming very great, and for example, where the design is such that automatic focusing by a mechanism such as a motor or the like is accomplished, the moving mechanism for the in-focus unit becomes bulky, and this has been disadvantageous for automatic focusing. In contrast, where the focusing systems other than the forward group axial movement system are generally adopted, the amount of movement for the in-focus varies in conformity with a variation in the focal length, and this has led to the problem that the focusing mechanism becomes very complex and structurally difficult.
As a solution to this problem, there has been proposed a construction as disclosed in Japanese Laid-Open Patent Application No. 4018/1982 wherein a new cam for focusing which operatively associates a zooming mechanism with a focusing mechanism is provided to thereby structurally enable focusing even if the amount of axial movement varies in conformity with a variation in the focal length.
On the other hand, as an entirely different solution, there is a construction in which the lateral magnification of the in-focus unit varies with zooming so that the amount of axial movement of the in-focus unit is substantially constant irrespective of the focal length. As an example of this, there is a construction as disclosed in Japanese Laid-Open Patent Application No. 202416/1983 wherein three in-focus units are moved as a unit, discretely from during zooming, and substantially by the same amount for any focal length, to thereby enable the in-focus to be accomplished, and there are constructions as disclosed in Japanese Laid-Open Patent Application Nos. 211117/1983 and 211118/1983 wherein two in-focus units are moved at different ratios discretely from zooming, to thereby enable the in-focus to be accomplished.
As the systems other than the forward group axial movement system, the above-described focusing systems may all be said to be systems of high realizability which solve the various problems peculiar to the focusing by the axial movement of the forward group. However, in any of these systems, the movement of the in-focus unit during focusing differs from the movement of the in-focus unit during zooming and therefore, it is necessary to provide an entirely different mechanism for enabling focusing and thus, the construction has unavoidably been complex.